The present invention relates to a process for making a joint between two metal wires of very small size and measuring devices obtained on the basis of this joint.
More specifically, the present invention relates to means for obtaining a joint between a wire made from a first metal and a wire made from a second metal corresponding to a very small junction length between the two wires, for example less than 10 microns.
It is known that in nuclear reactors, it is necessary to measure the thermal neutron flux distribution along the vertical axis of the reactor core. This applies inter alia to pressurized water reactors. The method consists of analysing the distribution of the flux using devices which are conventionally called gamma thermometers. In actual fact, the gamma heating produced by the neutron flux is measured. This analysis is carried out on the basis of the evaluation of the thermal conductivity in a stainless steel bar by means of differential thermocouples distributed along said bar, which substantially occupies the entire height of the reactor core. It is necessary for these bars to have a reduced diameter and consequently it is a fortioti necessary for the differential thermocouples to also have a very small diameter. More specifically, it is very desirable for the external diameter of the thermocouple, including its electrical insulating members to be approximately 1 to 0.5 mm. It is also known that in these reactors, it is important to carry out neutron flux measurements. A known neutron flux measuring device consists of an active platinum wire which behaves like a .beta. collector. This platinum wire is connected on the one hand by a weld to a conductive wire, e.g. of stainless steel, and on the other to the conductive sheath. In this case, it is also important to have a connection of minimum length between the platinum wire and the stainless steel wire. In fact, there is a mixture of platinum and stainless steel in the connecting zone. The presence of the platinum-alloyed stainless steel substantially modifies the .beta. collection properties of this second member. Thus, there is also a disturbance to the neutron flux measurement.
The attached FIG. 1 diagrammatically shows a differential thermocouple. This thermocouple generally comprises two welds S.sub.1 and S.sub.2 between a conductive wire F.sub.1 made from alumel and two conductive wires F.sub.2 and F'2 made from chromel. If the two welds S.sub.1 and S.sub.2 are placed in media corresponding to different temperatures .theta..sub.1 and .theta..sub.2, electromotive forces are produced at the said two welds. These are called e.sub.1 and e.sub.2. As a result of this differential thermocouple, it is possible to measure at the terminals A and B of the device the electromotive force e=e.sub.1 -e.sub.2 which therefore corresponds to the temperature difference .theta..sub.1 -.theta..sub.2 associated with the two welds S.sub.1 and S.sub.2.
Although the principle of differential thermocouples is well known, whilst their construction is also generally known, it is much more difficult to produce differential thermocouples, whose diameter is compatible with the requirements necessary for placing said thermocouples in a stainless steel bar for heating measurements due to the gamma radiation in a nuclear reactor core.
Thus, to obtain a precise measurement of the gamma heating, it is absolutely necessary for the weld between the chromel wire and the alumel wire or the welds between said two wires to have only a very limited length in order to ensure a very good localization of the measurement performed by this device.
A known method for obtaining differential thermocouples with an external diameter of the order of magnitude indicated hereinbefore (0.5 to 1 mm) consists of starting with a blank of the type shown in FIG. 2. This blank comprises a chromel wire 2 and a chromel wire portion 3 welded to one end of an alumel wire portion 4 by a weld 5. These two wire systems have a diameter of approximately 1.5 mm. The system of two wires is embedded in a ceramic insulating material formed, for example, by alumina or pulverulent magnesia, the insulant carrying the reference numeral 6. The external stainless steel sheath 8 surrounds the ceramic insulant. This blank has an external diameter of approximately 13 mm. The blank is subject to a drawing operation, which makes it possible to reduce the initial external diameter of 13 mm to a final diameter of 1 or 0.5 mm. The problem involved in this operation is that due to the drawing, even if the weld 5 is produced with maximum precision by known means, it is very significantly elongated.
Thus, in the case of wires having an initial diameter of 1.5 mm on starting with a 0.5 mm long weld of the initial blank (which already corresponds to a weld with a completely satisfactory quality) a very significant elongation of the weld is obtained during drawing in order to obtain the desired final size.
More specifically, if it is desired to obtain a thermocouple having an external diameter of 0.5 mm in its final state weld 5 has a length of 140 mm. On making do with an external diameter of 1 mm, the weld length is still 35 mm. However, such weld lengths are incompatible with a precise measurement of the gamma heating, due to the imprecision of the localization of the weld ensuring the measurement.